1. Field of the Invention
The present invention relates to an optical fiber manufacturing apparatus and an optical fiber manufacturing method.
Priority is claimed on Japanese Patent Application No. 2010-105216, filed on Apr. 30, 2010, the contents of which are incorporated herein by reference.
2. Description of the Related Art
An optical fiber is manufactured in a manner such that a silica glass rod called an optical fiber preform supported in a suspended manner inside a heating furnace is moved down, a front end thereof is heat-melted while it is moved down, and the optical fiber is drawn from the melted portion. At this time, in the optical fiber preform, an end portion which is not used for the fiber drawing operation is used as a dummy portion, and the dummy portion is supported in a suspended manner. The optical fiber preform may be manufactured by a method such as VAD method, OVD method, MCVD method, PCVD method, or RIC method.
On the other hand, in the heating furnace performing a melt-drawing operation, generally, the internal temperature increases up to about 2000° C. For this reason, a carbon-based material having heat resistance is used as a member of an interior portion of the heating furnace. However, although the carbon-based member has heat resistance, the carbon-based member deteriorates due to oxidization since it is exposed to a high temperature under the presence of oxygen gas. In this case, the interior of the heating furnace is contaminated by soot or the like formed by oxidization. When the contaminated state is left as it is, the strength of the manufactured optical fiber may be degraded.
Therefore, in order to obtain the optical fiber with satisfactory quality, the interior of the heating furnace needs to be maintained under the presence of inert gases such as nitrogen, argon, and helium, and the atmosphere (external air) needs to be prevented from intruding into the heating furnace.
In order to prevent external air from intruding into the heating furnace, it becomes important to maintain air-tightness at an opening portion located at the upstream of the heating furnace while the optical fiber preform is inserted through the heating furnace. Then, at this time, a variation in the outer diameter of the optical fiber preform in the longitudinal direction particularly becomes a problem. More specifically, a difference in the outer diameter between the dummy portion of the optical fiber preform and the rest of the body portion used for the fiber drawing operation, and a variation in the outer diameter at the boundary area between the dummy portion and the body portion are considered problems. This is because the air-tightness between the optical fiber preform and the opening portion of the heating furnace is difficult to maintain while the optical fiber preform having an uneven outer diameter is moved (moved down) in the longitudinal direction thereof.
As a method of preventing external air from intruding into the heating furnace, Japanese Unexamined Patent Application, First Publication No. H3-37128 and Japanese Unexamined Patent Application, First Publication No. 2005-225733 disclose a method in which a chamber blocking an entire optical fiber preform from external air is disposed on an opening portion located at the upstream of the heating furnace. However, in such a method, since there is a need to further provide a higher space above the heating furnace, a problem arises in that a large optical fiber preform or a high-speed fiber drawing speed is difficult to handle. Further, when the stream of a gas locally changes greatly inside the heating furnace, a problem arises in that the outer diameter of the optical fiber changes. However, in the existing method, no consideration is made to solve such a problem.
On the other hand, Japanese Unexamined Patent Application, First Publication No. 2009-62265 and Japanese Unexamined Patent Application, First Publication No. 2006-248842 discloses a method in which a seal member is provided on the opening portion located at the upstream of the heating furnace to contact the outer peripheral surface of the optical fiber preform, so that external air is prevented from intruding into the heating furnace.
However, in the method disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-62265 and Japanese Unexamined Patent Application, First Publication No. 2006-248842, there is a process of introducing an inert gas into the heating furnace while the optical fiber preform is moved down so that the melted optical fiber of the optical fiber preform is drawn. In such a process, since the stream of the gas locally changes greatly inside the heating furnace interior as described above, a problem arises in that the outer diameter of the optical fiber changes. Further, in the method disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-248842, when the optical fiber preform having an uneven outer diameter is moved down, it is difficult to maintain air-tightness. Accordingly, external air intrudes into the heating furnace, so that the stream of the gas locally changes greatly or the gas is mixed with oxygen, thereby causing a problem in that the carbon-based member deteriorates.
The present invention is made in view of such circumstances, and provides an optical fiber manufacturing apparatus and an optical fiber manufacturing method capable of preventing external air from intruding into a heating furnace and preventing a large variation in the local stream of a gas in the heating furnace.